Upon cell differentiation, many cell types exhibit polarized cell growth to form specialized structures which are crucial for the particular functions of the cells. I plan to investigate the mechanisms of polarized growth in mating cells of the yeast, Saccharomyces cerevisiae, which is readily amenable to classical and molecular genetics, and is a useful organism to study polarized cell growth. Haploid yeast cells when exposed to mating pheromones arrest cell growth and form a projection on one edge of the cell. Projection from two mating partners grow toward one another and fuse. The orientation of projection growth toward the partner is directed by pheromone gradient levels. The carboxy terminus of Ste2p, a 7-transmembrane segment pheromone receptor with a large cytoplasmic tail, is important for directing mating projection orientation. To understand how the receptor tail directs polarized growth, I plan to precisely determine the sequences which are responsible for mediating orientation. The regulation of these sequences by phosphorylation, and their role in other aspects of mating will be investigated. Finally, I plan to identify the proteins which interact with the carboxy terminal orientation sequences to determine how they direct cells growth. These studies are expected to provide insights into the mechanism by which external signals direct cell morphogenesis in yeast. Because many aspects of this process resemble those observed in other systems, these studies are expected to provide basic insights into mechanisms by which eucaryotes cells arrest cell division and differentiate into specialized cell types.